At finite temperatures, electrons in solids are thermally excited in accordance with the Fermi-Dirac distribution. This electron thermal excitation obscures or nullifies many novel and technologically important phenomena in various electron systems. For example, it can wipe out the Coulomb blockade in single-electron systems [1,2] and deteriorate the efficiency of spin-valve effect in spintronic systems [3,4]. Electron thermal excitation can also significantly degrade the performance of more mainstream electronic devices. For example, it is the root cause of excessive power dissipation in metal-oxide-semiconductor field-effect transistors (MOSFET); the electron thermal excitation prevents a steep turning-on/off of electric current, limiting the subthreshold swing to ˜60 mV/decade at room temperature, causing excessive power dissipation [5-7]. These are just a few examples, but the negative effect of electron thermal excitation prevails in solid-state electron systems in general. Therefore, if there were a method that could enable manipulation of electron thermal excitation, a broad range of scientific and technological benefits would be expected.
Previous studies by others have demonstrated that it is possible to suppress electron thermal excitations and obtain low electron temperatures by utilizing discrete energy levels present in quantum dots. If electron transport is made to occur through a discrete energy level, it can serve as an energy filter (or thermal filter) since only those electrons whose energies match the discrete energy level are allowed to participate in the transport. This has been experimentally demonstrated using double quantum dot systems, in which the first quantum dot adjacent to the source electrode serves as an energy filter, passing only cold electrons to the second quantum dot [8-10]. In a similar manner, it has also been demonstrated that the discrete energy levels or superconducting energy gaps can be utilized for quantum cooling of electron gases through energy-selective electron tunneling [11-15]. Until now, studies have been focused on obtaining ultralow sub-Kelvin electrons and investigating their novel phenomena, while the entire system is cooled to cryogenic temperatures, typically less than 1 K [8-15], regimes not suitable for practical applications.